The circulatory system requires mechanisms that prevent blood loss, as well as those that counteract inappropriate intravascular obstructions. Generally, coagulation comprises a cascade of reactions culminating in the conversion of soluble fibrinogen to an insoluble fibrin gel. The steps of the cascade involve the conversion of an inactive zymogen to an activated enzyme. The active enzyme then catalyzes the next step in the cascade.
Coagulation Cascade
The coagulation cascade may be initiated through two branches, the tissue factor pathway (also “extrinsic pathway”), which is the primary pathway, and the contact activation pathway (also “intrinsic pathway”).
The tissue factor pathway is initiated by the cell surface receptor tissue factor (TF, also referred to as factor III), which is expressed constitutively by extravascular cells (pericytes, cardiomyocytes, smooth muscle cells, and keratinocytes) and expressed by vascular monocytes and endothelial cells upon induction by inflammatory cytokines or endotoxin. (Drake et al., Am J Pathol 1989, 134:1087-1097). TF is the high affinity cellular receptor for coagulation factor VIIa, a serine protease. In the absence of TF, VIIa has very low catalytic activity, and binding to TF is necessary to render Vila functional through an allosteric mechanism. (Drake et al., Am J Pathol 1989, 134:1087-1097). The TF-VIIa complex activates factor X to Xa. Xa in turn associates with its co-factor factor Va into a prothrombinase complex which in turn activates prothrombin, (also known as factor II or factor 2) to thrombin (also known as factor IIa, or factor 2a). Thrombin activates platelets, converts fibrinogen to fibrin and promotes fibrin cross-linking by activating factor XIII, thus forming a stable plug at sites where TF is exposed on extravascular cells. In addition, thrombin reinforces the coagulation cascade response by activating factors V and VIII.
The contact activation pathway is triggered by activation of factor XII (also factor 12) to XIIa (also factor 12a). Factor XIIa converts XI to XIa, and XIa converts IX to IXa. IXa associates with its cofactor VIIIa to convert X to Xa. The two pathways converge at this point as factor Xa associates factor Va to activate prothrombin (factor II) to thrombin (factor IIa).
Inhibition of Coagulation
At least three mechanisms keep the coagulation cascade in check, namely the action of activated protein C, antithrombin, and tissue factor pathway inhibitor. Activated protein C is a serine protease that degrades cofactors Va and VIIIa. Protein C is activated by thrombin with thrombomodulin, and requires coenzyme Protein S to function. Antithrombin is a serine protease inhibitor (serpin) that inhibits serine proteases: thrombin, Xa, XIIa, XIa and IXa. Tissue factor pathway inhibitor inhibits the action of Xa and the TF-VIIa complex. (Schwartz A L et al., Trends Cardiovasc Med. 1997; 7:234-239.)
Disease
Thrombosis is the pathological development of blood clots, and an embolism occurs when a blood clot migrates to another part of the body and interferes with organ function. Thromboembolism may cause conditions such as deep vein thrombosis, pulmonary embolism, myocardial infarction, and stroke. Significantly, thromboembolism is a major cause of morbidity affecting over 2 million Americans every year. (Adcock et al. American Journal of Clinical Pathology. 1997; 108:434-49). While most cases of thrombosis are due to acquired extrinsic problems, for example, surgery, cancer, immobility, some cases are due to a genetic predisposition, for example, antiphospholipid syndrome and the autosomal dominant condition, Factor V Leiden. (Bettina R M et al. Nature 1994; 369:64-67.)
Treatment
The most commonly used anticoagulants, warfarin, heparin, and low molecular weight heparin (LMWH), all possess significant drawbacks.
Warfarin is typically used to treat patients suffering from atrial fibrillation. The drug interacts with vitamin K-dependent coagulation factors which include factors II, VII, IX and X. Anticoagulant proteins C and S are also inhibited by warfarin. Drug therapy using warfarin is further complicated by the fact that warfarin interacts with other medications, including drugs used to treat atrial fibrillation, such as amiodarone. Because therapy with warfarin is difficult to predict, patients must be carefully monitored in order to detect any signs of anomalous bleeding.
Heparin functions by activating antithrombin which inhibits both thrombin and factor X. (Bjork I, Lindahl U. Mol Cell Biochem. 1982 48: 161-182.) Treatment with heparin may cause an immunological reaction that makes platelets aggregate within blood vessels that can lead to thrombosis. This side effect is known as heparin-induced thrombocytopenia (HIT) and requires patient monitoring. Prolonged treatment with heparin may also lead to osteoporosis. LMWH can also inhibit Factor 2, but to a lesser degree than unfractioned heparin (UFH). LMWH has been implicated in the development of HIT.
Thus, current anticoagulant agents lack predictability and specificity and, therefore, require careful patient monitoring to prevent adverse side effects, such as bleeding complications. There are currently no anticoagulants which target only the intrinsic or extrinsic pathway.